Sleeping gold

Gold deposits are becoming increasingly complex to treat and grades continue to deteriorate in known reserves. Today, approximately 15-20 per cent of the world’s current gold production involves refractory ores that must be pre-treated prior to downstream recovery by cyanidation.

 

Ores are refractory for many reasons but commonly because gold occurs as tiny inclusions or submicroscopic gold within a sulphide mineral matrix. This mineral matrix must be physically and chemically altered to liberate the gold for subsequent leaching.

 

Peter Flanagan, FLSmidth’s SVP Key Accounts and Commodities says there are many undeveloped gold deposits where the resource head grade is simply too low to be economically viable using current refractory processing technologies.

Breaking new ground

FLSmidth is pioneering a mechano-chemical pre-treatment process known as the Rapid Oxidative Leach (ROL) for refractory gold ores. Sally Rocks, senior R&D chemist believes her team has made an extraordinary breakthrough which will have a profound effect on the industry. 

 

"We have discovered an economically viable method to process low-grade stockpiles and low-grade refractory gold deposits," she states.

"Initially, we are targeting refractory gold-bearing iron sulphides where the gold is locked inside the sulphide mineral matrix and cannot be recovered without pre-treatment."

 

Unlike other refractory processing techniques that require ultrafine grinding or high temperatures and pressures, the FLSmidth ROL gold process uses the application of mechanical energy coupled with oxidation under atmospheric conditions.

 

The process relies on Stirred Media Reactors (SMRt) to accelerate the oxidation of sulfide minerals. "Other technologies have relied on ultrafine grinding to increase the surface area of the particles. While ultrafine grinding is sometimes effective, it also requires a lot of energy and thus incurs a very high cost. We have successfully engineered a new low-energy process without having to ultrafine grind," she continues.

 

In the ROL process, the abrasion of the particle surfaces which occurs when the SMRt is activated is balanced to match the leach rate of the particles. Sally Rocks explains: "Judicious use of mechanical energy allows us to accomplish chemical reaction rates that are otherwise impossible without the use of high temperatures or pressures. The end result is a process that uses simple equipment and low-cost operating conditions for refractory gold pre-treatment."

 

She points out that the ROL process is still under development but initial batch studies have shown very positive results. FLSmidth is now working with several gold customers, to develop the process on a more significant scale.

 

 

Limitations of current technology

At present, the main refractory gold processing methods include ultrafine grinding, pressure oxidation (POX), roasting, or bioleaching.

 

Pressure oxidation, roasting and bioleaching have been successful in oxidizing refractory sulphide minerals to expose gold in solid solution that cannot be recovered by ultra-fine grinding alone.

 

However, POX and roasting have high capital intensity due to the extreme operating temperatures and pressures, exotic materials of construction and the ancillary equipment required to provide reagents and environmental controls.

 

Bioleaching operates at low slurry density and has comparatively longer residence time, often several days, which inflates the size of the leaching circuit. Bio-oxidation is also very sensitive to cyanide and thiocyanate as they are toxic to bacteria, so it is necessary to keep the pretreatment and cyanidation systems separate.

 

Pressure oxidation and roasting have high processing costs. Although processing costs can vary widely depending on power and reagent consumptions associated with sulphide oxidation, recent industry information indicates POX ranges between $50-65 per ton and roasting about $25-35 per ton. Peter Flanagan states that processing costs are typically the highest component of plant operating costs for refractory deposits. This means that at a gold price at $1250/oz gold, these current methods need 0.6-1.6 g/t Au just to cover processing costs. Many miners are faced with the dilemma that their refractory gold deposits are either too small to justify the capital outlay for current refractory processing technologies, or too low grade to cover the operational costs, or both.

Increasing economically mineable reserves

FLSmidth is working closely together with miners to unlock value, develop more productive operations, improve energy efficiency, reduce consumables consumptions, and lower operating costs, Peter Flanagan says. "The industry continues to focus on increasing margins through optimisation initiatives, and the ROL pre-treatment is a technology we believe has the potential to unlock significant value in the gold space. When you couple low operating cost with low capital intensity, it has significant impact on the asset Net Present Value," he explains.

 

"Due to the high capital intensity and high processing costs of current refractory processing methods, only assets with long mine life and high grades are achieving investment hurdles. There are a lot of undeveloped refractory gold deposits that have less than 3 g/t gold head grade, and some are coupled with small resources which translates into short mine lives. Currently those assets have little to no value as greenfield deposits and they need a step change in technology to unlock value," he concludes.

 

Industry data on known refractory gold deposits show that the amount of gold contained in refractory measured and indicated (M&I) resources is approximately double the contained gold in refractory gold reserves. Flanagan says, some portion of those M&I resources did not meet cut-off grade using current processing methods. These are deposits that are sufficiently drilled and some portion of the M&I resources would convert to economically mineable reserves at a lower cut-off grade.

 

Large deposits are becoming increasingly scarce. Since 2012, there have been less than 10 major gold deposits discovered globally. Looking at the five years prior to 2012, the number of major discoveries were nearly four times as many. While not all of these discoveries where refractory this is an indication that there isn’t the abundance of large gold deposits left, so the industry needs to find ways to treat smaller deposits going forward.

 

Extending mine life

FLSmidth's ROL process is not merely applicable to new deposits. According to Flanagan, there could be significant benefits to existing operations as well: "Many existing refractory operations have low grade stockpiles that are waiting to be processed at the end of the mine life. If you add ROL pretreatment to an existing operation, it could become viable to process low grade stockpiles earlier, rather than processing them at the end of the mine life. By reducing the cut-off grade, you can move those ounces forward to generate cash flow earlier and this will increase the asset Net Present Value."

 

The FLSmidth ROL technology could be a game-changer for the gold industry displacing current refractory processing methods, Flanagan says. "When we have proven this technology successful, miners will be looking at a completely different life of mine plan; by lowering the cut-off grade, more ore will be converted into reserves, extending mine life."